It is quite often desirable to provide an indicator light to show when a load circuit is energized. This is often true in a home or office, especially when the load device, such as a lighting circuit or other appliance, is of a type or in a location such that it is not visually apparent at the switch location when the load is energized. For example, a lighting circuit for an attic or other seldom used location can be left energized for long intervals of time if, as is true in many homes, the switch is located in a hallway or the like but the lights themselves are not visible when the access door is closed. This results in considerable wasted energy and can even create a fire hazard.
It is well known to provide a switch unit having a pilot or indicator light built in so that the indicator light is energized whenever the switch is in the "on" position. However, for such units to be usable in accordance with building codes, special wiring provisions must be made because, normally, the switch box does not have a neutral wire in it. Depending on location, it may be expensive and difficult to run extra wire in new construction to provide the neutral. In the situation where a switch unit with an indicator is being installed in existing construction, running new cable to provide the necessary neutral can be prohibitively expensive and also difficult.
It would be theoretically possible to connect a pilot light between the hot wire and the grounded wall box. However, such a connection would create a serious hazard because of the substantial ground current created and would be in violation of every known building code. Such codes normally do not permit the ground wires to carry load current, but it is generally recognized that certain types of loads, such as heater wires and the like, do have leakage currents to ground. These leakages are generally limited to 0.5 ma in hospitals and about 5 ma in other applications. However, intentional ground wire load currents of those magnitudes associated with equipment would not be sanctioned by Underwriters Laboratories. Currents at the 0.5 ma level, even though too high for acceptability, would not illuminate an indicator light of a type which could be seen under most ambient light conditions. As a result, indicator lights on switches are used rather rarely. Responsible agencies such as UL have indicated that intentional ground wire load currents substantially below the 0.5 ma level might be viewed as negliglible and acceptable.